1. Field Of The Invention
The present invention relates to alloy solders comprised of tin and copper that also may contain small amounts of silver, selenium and/or bismuth. These tin alloy solders can be utilized in applications where low toxicity, low application temperatures and high solder strength under cyclic load conditions are important, such as in heat exchanger construction and in electrical circuitry connections.
2. The Prior Art
Currently, lead based alloy solders are used almost exclusively for connecting tubes to header plates in the construction of heat exchangers. Normally, the tubes and header plates used in constructing heat exchangers are comprised of copper and zinc alloys, i.e. brass. When fluids used in heat exchangers contact the joints and seals created from the lead solder, the lead solder undergoes a chemical reaction or "bloom". This "bloom" is a result of chemical decomposition of the lead solder that increases when the fluids are not properly treated with the proper corrosion inhibitors. Lead from this decomposition will contaminate those fluids coming into contact with the lead solder. In addition, byproducts of the decomposition may cause plugging of heat exchanger tubes and will eventually weaken the joints and seals formed by the lead solder, causing them to fail.
The present invention addresses these problems by employing tin alloy solders comprised of tin and copper and that may contain small amounts of silver, selenium and/or bismuth. The tin solders of the present invention are not as susceptible to corrosion as are lead based solders. Also, because the tin solders contain no lead, they are less toxic to manufacture and to use than are lead based solders and, therefore, are also less costly than lead based solders from the standpoints of health protection and environmental compliance. Another advantage of the tin solders is that they are lighter in weight than lead based solders, thus reducing raw material shipping costs and product distribution costs. These cost reductions help offset the higher material costs for the metals that comprise the tin alloy solders.
Another problem with prior art joining welding is that, due to their higher application temperatures, they cause dezincification in the base metals, usually copper/zinc alloys. Dezincification occurs where the composition of the base metal is altered in the vicinity of where welding occurs. This happens because the zinc concentration in this area is decreased as a result of the zinc contained in the base metal alloy becoming liquified and moving away from the area of the solder prior to the liquefaction of the other components of the base metal alloy. Weakened solder joints result when dezincification of the base metals occurs.
By having a lower application temperature, the solder of the present invention avoids dezincification of base metals resulting in increased fatigue strengths, a feature especially important in applications involving cyclic loads of both thermal and vibrational natures. For example, when the solder of the present invention is used to construct heat exchangers, tube-to-header joint fatigue strengths are increased from 10 to 30 times over conventional lead solders, while tank-to-header plate fatigue strengths are increased 10 to 25 times over that of lead solders.
U.S. Pat. No. 4,758,407 reveals a lead-free, tin based solder composition for use in plumping applications for joining copper tubes and brass pipe and fittings. That invention is designed for applications where the solder is exposed to potable water and is designed so that the solder will not contaminate the water with lead, a known human toxin.
U.S. Pat. No. 4,758,407 includes claims for solder compositions including tin, copper, and silver. However, in addition to these three metals, each of the compositions revealed in U.S. Pat. No. 4,758,407 include either antimony or nickel or both antimony and nickel as additional components. The present invention includes tin and copper and optionally silver, but it does not include either antimony or nickel as additional components. Instead, selenium and/or bismuth are optionally added to the other metals to form the lead-free metal solder of the present invention. It would not be obvious to someone familiar with this area of art to eliminate the components of nickel and antimony from the alloy of U.S. Pat. No. 4,758,407 and add instead silver, selenium and/or bismuth.